Cambrian: Opinion

Elephant seals don't have many predators — but the ones they do attack from the deep

Elephant seals are top predators — of fish and squid. But they in turn are prey to even bigger predators: great white sharks and orcas. Sharks have been in the news lately, what with more than a dozen sighted from the air along the Pismo Beach shore. Many seals at Piedras Blancas show the scars of their encounters with critters big enough to eat them.

Smaller sharks also prey on seals, but not fatally. Cookie cutter sharks are small, less than 2 feet long. They attack with sharp teeth, twirl around and leave with a round plug of blubber.

They leave the characteristic cookie-shaped scars often seen on seals at Piedras Blancas.

Sharks eat plenty of seals, but not so much along the Central Coast. Sharks take what they can get, but their feeding is focused to the north, in Tomales Bay and the Farallon Islands in the fall.

Sharks are visual, stealth predators that attack from below and behind. They take a bite, then withdraw to allow their prey to weaken.

In the spring, white sharks migrate out to the White Shark Café, out in the open ocean between Baja and Hawaii, the edge of the North Pacific Subtropical Gyre. Scientists are out there now, tracking the sharks to figure out what they are doing there. There’s not as much prey there as the California coast offers.

“In the café itself — think empty blue — we have almost no measurements of these biologically essential parameters,” the team posts on its website, schmidtocean.org/cruise/voyage-white-shark-cafe/#about. “So the team simply does not have the measurements and observations to understand what that food source could be or why such a region would attract a large apex predator.”

Biologging tags transmit data about where the sharks are. At night, they dive 300-600 feet, turn around and come right back up, a V-shaped dive. During the day, they dive far deeper, 1,500 feet. Barbara Block and her Stanford University Hopkins Marine Station team are watching over them. Are they eating down there, or does it have to do with mating?

They tagged 37 sharks last fall, programmed to release from the sharks in April and May. The team is out there now, on the Research Vessel Falkor to collect them. They’ll also be listening for pings from 80 other sharks that have been tagged over the past four years.

The team is using new high-tech devices to help study the sharks. A Slocum Glider drone with shark-tracking equipment was sent in March, to arrive in time to greet the research team. They’ll use two Saildrones, unmanned surface vehicles that “sail” on a fixed wing, to collect data on plankton and midwater fish. They’ll also collect bits of DNA floating in the water to analyze what has been swimming through.

Christine Heinrichs’ column appears the fourth Thursday of each month and is special to The Cambrian.
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